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Backup generator sizing?

darkskies

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Dec 3, 2022
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59
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New Hampshire
Hi,

Can anyone point me to information on how one determine the "proper" sizing of a whole house generator used as a backup for an on-grid PV system?

I've seen some on-line calculators, but they seem very rough calculations. I didn't know if there was a better way to determine the size.
Can I take advantage of the consumption data from my PV system to help in determining the sizing? It shows the power consumption in 15 minute intervals via a graph, and I have daily/weekly/yearly power consumption information as well.

Thanks in advance!
 
Make an itemized list of all your big-ticket electrical users. Refrigerator, washer/drier, electric stove, hot water heater. Get a least 2X the generator power for whatever your single largest load you want to run is.
 
Hi,

Can anyone point me to information on how one determine the "proper" sizing of a whole house generator used as a backup for an on-grid PV system?

I've seen some on-line calculators, but they seem very rough calculations. I didn't know if there was a better way to determine the size.
Can I take advantage of the consumption data from my PV system to help in determining the sizing? It shows the power consumption in 15 minute intervals via a graph, and I have daily/weekly/yearly power consumption information as well.

Thanks in advance!
If you size your Solar system correctly you shouldn’t need a very big generator.
Saying that We have had an unusually bleak December so far.
 
I have a larger off-grid (with grid assist) solar system for my home. December is brutal as I only get ~500kwh/month compared to 2000kwh/month in spring/summer. I need at least 1200kwh/month to run my home. That's a 1200kwh need - 500kwh actual = 700kwh shortage. 700kwh / 31days = 22.6kwh/day shortfall

My system is large enough I could handle up to 20,000w of charging but.... yikes that's too large a generator + fuel + noise + chargers + wiring for a home in the city with neighbors.

So I'm shooting for 8000w continuous output propane generator for charging as these are pretty well priced / available. I'm imaging this will get me in the range of 3hrs run-time for 23kwh. 6hrs run-time would get me 2 days worth of charge. Also this would be enough to power a lot of the house directly in the event of battery maintenance.

Sizing a generator (for me) involves reasonable sizing per what you can buy / run for you're situation to meet you're needs. For example, I have a Honda eu3000is but at 2500w output, 23kwh/day would be 9hrs/day run-time - e.g. barely cover my needs / a lot of personal attention-supervision day after day / a lot of wear on the generator in terms of run-time hours. Seems like a 8000w would be better suited for the level I want to cover.
 
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Hi,

Can anyone point me to information on how one determine the "proper" sizing of a whole house generator used as a backup for an on-grid PV system?

I've seen some on-line calculators, but they seem very rough calculations. I didn't know if there was a better way to determine the size.
Can I take advantage of the consumption data from my PV system to help in determining the sizing? It shows the power consumption in 15 minute intervals via a graph, and I have daily/weekly/yearly power consumption information as well.

Thanks in advance!
Lets say you have a critical loads panel that is energized by a inverter/charger or all in one.
When the battery needs to be re-charged and the sun won't cooperate then you start the generator.
The inverter/charger or all in one goes into bypass mode and the generator now has to cover your critical loads and re-charge your batteries.
Prevailing wisdom is to not load your gasoline or propane generator to more than 2/3rds of its running watts rating.

My little gasoline inverter/generator is rated for 2000 starting watts and 1500 running watts.
1500 running watts * .66 de-rate = 990 watts.
That means both my loads and battery charging should not exceed 990 watts.
Lets call it 1000 watts.
My baseline critical loads is < 200 watts so I can put 800 watts into battery charging.
I estimate my charger is 80% efficient so I would be putting ~640 watts into the battery.
Because my generator has a small tank I need to cold fuel and then hot fuel every day that grid and sun are not available.

Forum wisdom says a diesel generator can be loaded much higher.
Maybe 80-90%.
 
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If you size your Solar system correctly you shouldn’t need a very big generator.
Saying that We have had an unusually bleak December so far.
We live in an area where there are occ.asional outages in the Winter, and being near the end of the line, power can be out for several day. THat and we are on well.
 
Hi,

Can anyone point me to information on how one determine the "proper" sizing of a whole house generator used as a backup for an on-grid PV system?

If you have an on-grid PV system meaning grid-tie (only) PV, that is a red herring. Size the generator to your loads. Have GT PV disconnected while operating the generator.

If you are trying to use PV system size to see what consumption is, it may equal average over the year, rather than peak consumption. Or it may be size smaller than average consumption, leaving some use of utility power.

If you have a hybrid (or off-grid) inverter, you may be able to feed it from a generator and take advantage of PV as well. The generator has to be stable enough for the PV to synch up.

Some inverters, they require a generator larger than inverter capacity. Others, including Sunny Island, can work with a very small generator, e.g. drawing as little as 1200W from generator while adding 11kW from battery to feed load.

Depends entirely on the features and performance of your inverter.

We live in an area where there are occ.asional outages in the Winter, and being near the end of the line, power can be out for several day. THat and we are on well.

You need a generator (or battery inverter) capable of delivering surge current to start well pump. Typically 5x nameplate current.
 
We live in an area where there are occ.asional outages in the Winter, and being near the end of the line, power can be out for several day. THat and we are on well.
Do you happen to have a tractor with a pto shaft?
Maybe a pto generator would work for you.
 
Make an itemized list of all your big-ticket electrical users. Refrigerator, washer/drier, electric stove, hot water heater. Get a least 2X the generator power for whatever your single largest load you want to run is.
I guess the central air and well are the biggest draws, followed by fridge and freeze, but I'll have to see what they actually draw.
 
I have a larger off-grid (with grid assist) solar system for my home. December is brutal as I only get ~500kwh/month compared to 2000kwh/month in spring/summer. I need at least 1200kwh/month to run my home. That's a 1200kwh need - 500kwh actual = 700kwh shortage. 700kwh / 31days = 22.6kwh/day shortfall

My system is large enough I could handle up to 20,000w of charging but.... yikes that's too large a generator + fuel + noise + chargers + wiring for a home in the city with neighbors.

So I'm shooting for 8000w continuous output propane generator for charging as these are pretty well priced / available. I'm imaging this will get me in the range of 3hrs run-time for 23kwh. 6hrs run-time would get me 2 days worth of charge. Also this would be enough to power a lot of the house directly in the event of battery maintenance.

Sizing a generator (for me) involves reasonable sizing per what you can buy / run for you're situation to meet you're needs. For example, I have a Honda eu3000is but at 2500w output, 23kwh/day would be 9hrs/day run-time - e.g. barely cover my needs / a lot of personal attention-supervision day after day / a lot of wear on the generator in terms of run-time hours. Seems like a 8000w would be better suited for the level I want to cover.
At the previous home, I had a Honda 5500W gasoline powered unit. It only provided critical circuits (as was a later add-on with manual transfer switch). It was a PITA refilling it with 5 gallon cans.
 
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It's getting late here, but I'll try to give a complete picture of what I'm trying to figure out, given what I have in place already, and what I'm hearing from the solar system installer and manufacturer (which is changing w.r.t. the interconnection of the generator).

I'm not a DIYer, but am trying to learn more about PV systems, so that I can make sure things are being done well.
 
I guess the central air and well are the biggest draws, followed by fridge and freeze, but I'll have to see what they actually draw.

Seems to me a wire from well pump switch cutting out A/C might reduce needed generator (and inverter) size in half.
Actually, unless both tried to start at same time you wouldn't need double starting surge. But should still help.

Specs, or inrush readings taken with a clamp ammeter having that feature, would help.

Installers will sell what they have or they know. Useful to buy something they are able to service. But also good to understand specs and reputation, make sure what you're being sold should meet your needs.
 
Here's some (longish) history on the project, so that folks can see where I'm coming from and what I'm trying to figure out...


Goals...

This is my second home with solar PV. I'm using the same installer, who did a great job on the first home, both in workmanship and quality of products. I was very pleased with the install and ran the system for 7-8 years, before selling the house. With this house, I was looking to do two things different (of which I'd have the installer provide)...

1) Generator
I wanted an automatic whole house generator for backup power. Our previous home would loose power a few times each Winter, mostly for a few hours, but occasionally a few days have occurred over the years. With our current home (in the same rural town, but at the end of the line for utility company lines), we are expecting (and have heard from neighbors) that outages are more common.

Given we are older, and my wife can't haul around 5 gallon containers of gasoline, I wanted to have a propane generator with automatic switchover. We also wanted whole house, instead of critical circuits, mostly because of convenience (years after we had our previous generator, we discovered we needed other/more circuits covered - could have done some rewiring, obviously, but it was a pain having to run extension cords to keep a UPS running in the study, or power a CPAP machine in a bedroom).

2) Battery
I wanted to consider battery backup, mostly as a long term cost savings measure, given that propane is expensive here. The thought was that for most outages, the battery would handle things (and save on fuel), but we'd still have the generator for those extreme outages.

Design...

We used the same installer, as were happy with the work they did before, and they are familiar with our previous system, power consumption, etc.

The designed system was 11.9 Kw of panels (up from 8.55 kW at previous home) with a SolarEdge 7.6 kW inverter (just like our previous home). They would use the "new" Energy Hub and Backup Interface that SolarEdge provided, which has support for EV charges, batteries, multiple inverters, etc. They had the option of the 10 kWh LG battery or the 16 kWh LG that was just becoming available.

Given the hub could support only up to two batteries and the difference in price between the two, we decided to go with the 16 kWh battery. As I understood it, the batteries are charged without using the inverter, so excess power from the panels can charge the batteries without affecting the power that the inverter can provide.

They had spec'ed out a Generac 10 kWh propane generator for the project as well. I asked about the size of the generator (as people I know have 15-20 kWh ones, although with much larger houses), and was told that they way the system would work is that, when the grid fails, the house would be powered by battery (and PV, if available). When the battery gets low, the generator would be used to "recharge" the battery. I was convinced that was fine.

It was more costly than we had expected for system cost, but this was to be our retirement home and we figured we'd splurge a bit and try to get something setup that would work for us, allow grown in the future (say with EV cars), and save us money operationally down the road.


Project...
Here's were things went awry...
The PV (only) system was installed in July of last year. The battery was on back-order due to newness and supply chain issues. The generator came in and was installed in November, and the installer connected in up to the backup interface. Without a battery, though, this did not work, and it was around that time that we (both the installer and I) discovered that although SolarEdge's data sheet said their inverter/hub/backup interface worked with a generator (and the reason why the installer went with that solution), but had a asterisk and in fine print said "with a required firmware update".

It turns out that the firmware was NOT available, and over time we kept getting different dates of availability Q4 2021, then Q4 2022. The installer, put in a sub panel with ATS and critical circuits routed to it, and connected the generator to that. At least through the Winter of 2021, we had a generator for power outages.

In August of this year, the battery finally came in, and was installed. Now, when the PV stops producing, the battery powers the house (another painful story - as it only draws down to 80% capacity and then stops - I've been nagging my installer and SolarEdge on how to get that so it draws down to 20%). When the power goes out, the generator comes on, and powers critical circuits.

Current Status...
So, I have all the pieces, but not integrated in. Well, I just found out from SolarEdge that the firmware is available. They had a release note dated in July (I've been nagging them every month asking about the firmware and it wasn't until now that I found out). I'm trying to get a date from my installer, for when they can install the firmware, and re-wire the system to get the final integrated solution.

However, after reading the release note, it appears that the way they suggest this all works is different than what my installer had said would occur (and the installer is referencing what they were told by SolarEdge). From what I can tell, is that the PV and battery are on the grid side, and would feed an ATS with the generator. When power goes out, we run on battery, and when the battery gets low, it switches to generator to (as I understand it) power the whole house. I'm not sure if the generator would also charge the battery.

I've mentioned this to the installer, who is going to look into it more.

I must admit, I've gotten a real sour opinion of SolarEdge from the misleading advertising of the firmware availability, and the horrible customer support I've received from them over the year. I'm not happy with my installer, mostly because they haven't stayed on top of things, and have been very unresponsive (their claim is they are flat out with installs due to rate increases lately - my response being that I've already paid in full for the system - which I did so that I could declare everything on 2021 taxes - a mistake I'm now regretting).

It raises two questions.

First, am I understanding the release note and how the system would operate, or is there some other configuration that would be used (like what was mentioned at design time)?

Second, if the generator is to run the whole house, is a 10kWh generator sized correctly (as that was not it's initial intent)?

Hence the subject of this thread.
 
I guess the central air and well are the biggest draws, followed by fridge and freeze, but I'll have to see what they actually draw.
Since most of your blackouts are in the winter, then I think it's OK to exclude the central air, which I know will be the single biggest load.

The well at my homestead is the single biggest load. It draws about 2000W while running. For years after we bought the property we were using an old Onan 4000W generator that was left behind. That successfully powered the well-pump for years, till it got old enough that the inrush draw of the pump started to stall it. I ended up replacing it with an AC Delco 6000W generator, which I still have.

So, I guess I'm not following my own recommendations, and went with 3X instead of 2X. How big is your pump? Unless you really need to run central air during a blackout, I'd say you'd do fine with something in the 5000-7500W range. Although refrigerators and freezers utilize a lot of power over the course of a 24hr period, their instantaneous usage is likely to be in the 200-300W range.
 
Here's some (longish) history on the project, so that folks can see where I'm coming from and what I'm trying to figure out...




Second, if the generator is to run the whole house, is a 10kWh generator sized correctly (as that was not it's initial intent)?

Hence the subject of this thread.
Based on your account I would say no. It might have been sized to charge the batteries when PV power was insufficient but it will not carry a heavy household load. Your installer needs to do a bit more planning.
 
Since most of your blackouts are in the winter, then I think it's OK to exclude the central air, which I know will be the single biggest load.

The well at my homestead is the single biggest load. It draws about 2000W while running. For years after we bought the property we were using an old Onan 4000W generator that was left behind. That successfully powered the well-pump for years, till it got old enough that the inrush draw of the pump started to stall it. I ended up replacing it with an AC Delco 6000W generator, which I still have.

So, I guess I'm not following my own recommendations, and went with 3X instead of 2X. How big is your pump? Unless you really need to run central air during a blackout, I'd say you'd do fine with something in the 5000-7500W range. Although refrigerators and freezers utilize a lot of power over the course of a 24hr period, their instantaneous usage is likely to be in the 200-300W range.
Yeah, I don’t expect to see outages in the Summer, and don’t care if there is no A/C.

We have a gas oven/stove, dryer, and H/W tank. Forced hot air for heat, with a mini split door a large finished room in the basement. Full sized refrigerator and a separate full size freezer.

Several computers (servers), microwave, garage door opener for a double wide door, several TVs, audio/video gear (one of my hobbies is home theater), typical lighting for a ~2200 sq ft 3 bedroom ranch, and the well pump.

I’ll have to research into the well pump as e are the second owners of the house. I think it is fairly large, about 400’ down and over 200’ from the house. Will see what I can find out.
 
Based on your account I would say no. It might have been sized to charge the batteries when PV power was insufficient but it will not carry a heavy household load. Your installer needs to do a bit more planning.
I’ve got the installer looking into the release notes to confirm that the generator would be driving the whole house (not what they designed for). If that is the case then we’ve got a larger issue to resolve.

From my perspective, it’s up to the installer to meet my original goals of whole house backup for the solar system, whether that be through a generator charging the battery or running the whole house.

I’m trying to do some research to see what the generator sizing should be, if it has to support the whole house.
 
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